Trends in micro electronics show an increase in power dissipation as well in power density. As a result the limits of air cooling for high performance applications are reached and those of low end applications will be reached in the coming years. In order to deal with this limit, a transition to liquid based cooling techniques is gradually made.
Different types of cooling are known. Cooling can be active or passive. In passive cooling, no additional power is used and the heat is transported by conduction and/or radiation heat transfer. Two examples of passive cooling are natural convection cooling (passive air cooling) and passive liquid cooling using heat pipes and vapor chambers. In active cooling, the fluid motion is assisted by an external source, such as a fan for a forced air cooled system or a pump in a liquid cooled system.
Conventional forced liquid cooling systems consist of a separate cooling unit mounted to the chip. This cooling unit contains channels parallel to the chip surface through which a coolant is pumped. As the liquid moves through the channels, it takes up the heat from the chip and the liquid heats up.
For this conventional liquid cooling approach, some drawbacks are known.
Since the liquid temperature increases along the length of the channel, the part of the chip at the end of the channel is cooled less efficiently than the part at the beginning of the channel resulting in a temperature gradient across the chip surface. This temperature increase is referred to as the caloric thermal resistance.
The presence of the adhesive, or thermal interface material (TIM), between the chip and the cooler results in an additional thermal resistance. In the case of a high performance liquid cooler, the contribution of this TIM thermal resistance can be significant or even dominating.
In such conventional liquid cooling approach, the required pressure also scales linearly with the length of cooling channels.
Some solutions were presented that try to deal with at least some of these problems. US patent application US2009/0084931, by Intel Corporation, discloses a liquid cooling device for cooling a die. The liquid cooling device comprises a support block supporting a plurality of vertical channels transporting fluid to and from a bare die surface for removing heat. The vertical channels are formed by parallel pipes, positioned adjacent and separated by walls.
In “Microjet Cooler with Distributed Returns”, Heat Transfer Engineering (2007) 28(8-9) p 779-787, Natarajan and Bezama describe a cooler made in ceramic material providing a liquid microjet array based on a manifold showing a plurality of inlet jets and outlet jets.
US patent application US2005/0143000, by Danfoss Silicon Power GmbH, a cooling device is described that comprises a heat conducting cooling plate on the side of the electronic power components to be cooled in combination with a platelike cooling fluid distributing device. The cooling device comprises a first plate in which outlets and a plurality of drain outlets are evenly distributed and further plates defining a feed channel and a drain channel connected to respectively the outlets and the drain outlets.